Differential linear amplifier and switching circuits are commonly used in electronic systems. FIG. 1 illustrates an exemplary differential circuit. The differential circuit includes a pair of transistors having the emitters connected to a common current source and the collectors connected to load resistors which are in turn tied to a voltage supply. In operation, a differential signal of opposite polarity may be applied to the bases of the transistors, thereby resulting in an amplified signal appearing at the collectors of the transistors. The bandwidth of the circuit is commonly defined as the frequency where the output amplitude drops by −3 dB. However, bandwidth is affected by a number of parameters, including the forward transit time of the transistors, junction and parasitic capacitances associated with the transistors, load resistance, inductance in the metal interconnect, etc.
For high speed applications, the collector time constant is a significant limitation on bandwidth. Collector capacitance acts as a shunt in parallel to the load resistor, thereby gradually reducing the output amplitude with increasing frequency. The pole set by the collector node can be moved out in frequency by reducing the collector load resistance and increasing the operating current. However, there are limits to these measures. Temperature rise causes transistor parameters to deteriorate as well as higher power dissipation.
Therefore, it is desirable to provide an improved load configuration for differential circuits which increases circuit bandwidth without increasing power dissipation.